If the skies are blue, as Jeff says, use a yellow, orange, or red filter or a polarizer. If the sky is hazy, use a graduated netural density filter. It will be darkened, but still won't have much detail.
For outdoor use, I would recommend a slower, finer grain film. Something like Ilford FP4 might be good. Relatively fine grain and higher contrast. If the contrast seems to high, pull it a little. It will give smooth skies and sharp detail. Do you even need the speed of Tri-X for portraits? Even there I would go with a slower, finer grain film.
My philosophy has always been, go with the slowest, finest grain film that will do the job. Unless you're looking for the grainy look, why make things difficult. Just IMHO.